Abstract: We describe a method to tune, in-situ, between transverse and longitudinal
light-matter coupling in a hybrid circuit-QED device composed of an electron
spin degree of freedom coupled to a microwave transmission line cavity. Our
approach relies on periodic modulation of the coupling itself, such that in a
certain frame the interaction is both amplified and either transverse, or, by
modulating at two frequencies, longitudinal. The former realizes an effective
simulation of certain aspects of the ultra-strong coupling regime, while the
latter allows one to implement a longitudinal readout scheme even when the
intrinsic Hamiltonian is transverse, and the individual spin or cavity
frequencies cannot be changed. We analyze the fidelity of using such a scheme
to measure the state of the electron spin degree of freedom, and argue that the
longitudinal readout scheme can operate in regimes where the traditional
dispersive approach fails.